First you might want to change your settings to Guru difficulty (to help obtain 40 Guru wins at the same time). You change the settings when you first start your game.
I found the games easiest to win with1 opponent (Lost about 1 out of every 4-5.games, which wasn't bad)
Domino set to 9 (aim for getting more points faster
Starting hand 7 (more chances to get big points early)
Winning score to 100
Spinner on
Or simply can boosted with a friend online and alternate wins.
You can check your progress by going to the Help & Options and going to My Stats